1. Field of the Invention
This invention relates to integrated circuit (IC) packages, and more particularly, to a heat-dissipating structure for use in an integrated circuit package, such as a BGA (Ball Grid Array) integrated circuit package, for heat dissipation from the integrated circuit package during operation.
2. Description of Related Art
BGA integrated circuit packages allow the integrated circuit package to have a low profile while nevertheless incorporating a large packing density of electronic components with a high package pin count. During the operation of the integrated circuit chip, a large amount of heat can be generated due to the flowing of electricity through the electronic components on the integrated circuit chip. If this heat is not dissipated, it can cause damage to the internal circuitry of the integrated circuit chip. Therefore, it is required to provide heat-dissipating means on the integrated circuit package for heat dissipation during operation.
Types of BGA integrated circuit packages include PBGA (Plastic BGA), CBGA (Ceramic BGA), and TBGA (Tape BGA), which are so named in terms of the material being used to form the substrate. Among them PBGA integrated circuit packages, however, are poor in heat-dissipating efficiency since plastics are poor in heat conductivity. To allow PBGA integrated circuit packages to have a high heat-dissipating efficiency, a conventional solution is to provide a heat sink or a heat slug.
The U.S. Pat. No. 5,736,785 to Chiang et al. discloses a BGA integrated circuit package that is provided with a heat sink. FIG. 5 shows the structure of this BGA integrated circuit package. As shown, the patent utilizes a heat sink 116 which is mounted on a substrate 104 on which an integrated circuit chip 102 is mounted. The heat sink 116 is formed with a circular downwardly-protruded portion 116a in the center thereof, which is abutted on the top side of the integrated circuit chip 102, allowing the heat produced from the integrated circuit chip 102 to be dissipated by the heat sink 116 to the atmosphere. Further, the heat sink 116 is formed with another four downwardly-protruded portions 116c which are dimensioned with a larger depth than the protruded portion 116a so that the entire body of the heat sink 116 can be supported on the substrate 104. The heat sink 116 is further formed with an upwardly-protruded portion 116d which is exposed to the outside of the integrated circuit package so that the heat can be dissipated to the atmosphere.
The use of the foregoing heat sink 116 for heat dissipation, however, has the following drawbacks. First, since the integrated circuit chip 102 is very fragile and small in size, the abutting of the heat sink 116 on the integrated circuit chip 102 would easily cause the integrated circuit chip 102 to crack during the cooling process of the encapsulate 112, or during the cooling process of the integrated circuit package after solder reflow, or during the temperature-cycle reliability test. This is because that the copper-made heat sink 116 has a significantly greater coefficient of thermal expansion (CTE) than the integrated circuit chip 102 (copper has a CTE of 18 ppm/.degree. C., while integrated circuit chip has a CTE of 3 ppm/.degree. C.); and therefore, during a cooling process, the heat sink 116 would cause a thermal compressive stress to the integrated circuit chip 102, thus causing the integrated circuit chip 102 to be cracked.
Second, since the resin-made encapsulant 112 has a large CTE of about 20 ppm/.degree. C. and encloses a large part of the active surface of the integrated circuit package (the term "active surface" refers to the surface part of the integrated circuit chip where electronic components are formed), it would also easily cause the integrated circuit chip 102 to be cracked or delaminated from the encapsulant 112 during the cooling process of the integrated circuit package.
Third, during molding process, the existence of the downwardly-protruded portion 116a, 116c would cause disturbance to the resin flow, thus causing undesired forming of voids in the resulted encapsulant 112. The forming of these voids would easily cause a popcorn effect during the solder-reflow process, which might result in damage to the integrated circuit package.
Fourth, the heat sink 116 is still considered unsatisfactorily low in heat-dissipating efficiency due to the fact that the upwardly-protruded portion 116d is relatively small in extent compared to the overall size of the heat sink 116; and therefore, the majority of the heat would dissipate through the following path: chip.fwdarw.encapsulant.fwdarw.heat sink.fwdarw.encapsulant.fwdarw.atmosphere. Since the resin-made encapsulant 112 has a heat conductivity of just 0.8 w/m.degree.K, the heat dissipation through this path would be undoubtedly low in efficiency.
One solution to the foregoing problems is shown in FIG. 6. This solution is characterized in the provision of an improved heat sink 116' which is unabutted on the integrated circuit chip 102'. This feature can prevent the resin flow during the molding process to be subjected to disturbance as well as prevent the heat sink 116' from causing a thermal compressive stress on the integrated circuit chip 102' during the cooling process. In addition, the heat sink 116' is formed with a large exposed area to the atmosphere, which can help increase the heat-dissipating efficiency.
One drawback to the forgoing solution, however, is that, since the integrated circuit chip 102' is entirely encapsulated in the encapsulant 112', the cooling process would nevertheless cause the encapsulant 112' to bring a thermal compressive stress to the integrated chip 102' during the cooling process. Moreover, the heat sink 116' is still considered unsatisfactorily low in heat-dissipating efficiency, since most of the heat from the integrated circuit chip 102' is transferred to the heat sink 116' via the encapsulant 112' and the encapsulant 112' is quite low in heat conductivity. Still moreover, the integrated circuit package shown in FIG. 6 is incapable of providing a strong mechanical strength to the integrated circuit chip 102', making the integrated circuit chip 102' vulnerable against the thermal compressive stress from the resin-made encapsulant 112 and the tensile stress from the silver-paste layer.
Furthermore, the above-mentioned two BGA integrated circuit packages respectively shown in FIGS. 5 and 6 would provide a penetrating passage to allow atmospheric moisture to penetrate through the encapsulant to the integrated circuit chips enclosed therein, which may cause the integrated circuit chips to function improperly during operation.